1. Field of the Invention
The present invention relates generally to a voltage controlled oscillator for a Radio Frequency (RF) integrated circuit and, more particularly, to a push-push voltage controlled oscillator that can obtain output signals having a frequency twice as high as the fundamental resonance frequency of an LC resonator with differential signals having the same amplitude and opposite phases.
2. Description of the Related Art
Recently, markets for mobile communication terminals that not only operate in several frequency bands but also have various functions have developed domestically and abroad.
Therefore, mobile communication terminals are being miniaturized in order to satisfy the demand of consumers, the parts of mobile communication terminals are being miniaturized, and the various parts of the RF system are also being miniaturized.
Particularly, in order to implement a transceiver for high frequency communication, an oscillator having a high output frequency, low phase noise and low power consumption is essential. Generally, inductors and capacitors, which are components constituting the LC resonator, must be physically miniaturized in order to make an oscillator with an LC resonator to oscillate in a high (radio) frequency band. Since it is known that, as elements become smaller, the errors of the elements increase, and there is a strong possibility that the error between a design frequency value and a manufacturing frequency value increases as the operating frequency of an oscillator increases.
In order to solve this problem a method of using a separate frequency doubler or a separate frequency multiplier in a voltage controlled oscillator is used for acquiring a high output frequency. However, this method has a problem in that an additional circuit is required and current consumption increases.
Another method of increasing an output frequency is based on the structure of a push-push voltage controlled oscillator. This structure is constructed so as to combine two outputs from two balanced oscillators at one point and thus obtain a doubled output frequency.
Meanwhile, it is known that differential signal processing is superior to single-ended signal processing in order to efficiently eliminate common-mode noise in an RF/analog Integrated Circuit (RFIC) and, thus, achieving signal processing resistant to noise. Therefore, RF/analog integrated circuit designers demand circuit structures capable of generating or processing differential signals. However, a conventional push-push voltage controlled oscillator generally generates only single ended output.
Accordingly, in order to acquire differential signals, an additional circuit, such as a single-to-differential converter, must be further included.
As illustrated in FIG. 1, a frequency doubler, which generates differential signals, includes a power supply (VDD), transistors M1, M2, M3 and M4, impedance elements LP and LM, a non-inverted input signal Vin+, an inverted input signal Vin−, and a ground terminal. Using this, differential signals composed of fundamental frequency components, which are generated by a voltage-controlled oscillator, can be converted into differential signals each having a doubled frequency. However, there is a problem in that an additional circuit, called a differential frequency doubler, is required which results in an increase in circuit complexity and power consumption.
Furthermore, in order to acquire a high frequency within a voltage-controlled oscillator without additionally using a frequency doubler, a push-push voltage-controlled oscillator may be used. The push-push voltage-controlled oscillator has an efficient structure capable of obtaining a frequency twice as high as a fundamental operating frequency. This has advantages in that current consumption is less than that in a general structure, and when it is applied to a direct conversion type RF transceiver, the undesired pulling problem experienced with a voltage-controlled oscillator decreases. As a result, the push-push voltage-controlled oscillator structure is widely used in a Si RF integrated circuit or a compound semiconductor Monolithic Microwave Integrated Circuit (MMIC). As illustrated in FIG. 2, the structure of a conventional push-push voltage-controlled oscillator includes an LC resonator which includes an inductor Ltank and variable capacitance diodes Ctank, and determines a fundamental oscillation frequency, and a transistor pair M1 and M2 which generates negative resistance in order to compensate for the loss of the LC resonator, thereby outputting a doubled frequency at a common source terminal M. In this case, in order to increase the amplitude of the output signal, an impedance element is provided to the common source terminal M. However, the structure has a problem in that there is only one output signal. As described above, a single signal is very sensitive to external noise, so that it is difficult to apply it directly to an RF integrated circuit.